Ernest Pogosyants was the most prolific endgame study composers, with more than 1,700 units. In total he composed more than 6,000 problems and studies. You may read his article in EG 56, June 1978, about Castling in studies. Tim Krabbe wrote a review of "A (First) Century of Studies", in which John Roycroft presents a first batch of 100 endgame studies by Pogosyants, and some appreciative words about Pogosyants' work in his chess diary ("Pogosyants' sleepless nights").